Resistant ryegrass continuing problem

Hoelon-resistant ryegrass continues to be a problem in Arkansas wheat production. Resistant genotypes of ryegrass have now been confirmed in all but one of the wheat-producing counties in Arkansas. This spread of resistant ryegrass has continued even though wheat production has been down over 300,000 acres statewide over the last two years.

Hoelon is in the same group of herbicides as Clincher, Poast and Select. To date, four different genotypes of ryegrass have been identified with varying levels of resistance to Hoelon and other ACCase-inhibiting herbicides.

Also, these four genotypes of ryegrass have been found in a wide range of different counties across the state. We learned that resistant populations have developed independently from continued use of Hoelon and also have been spread by seed or pollen movement. One biotype of ryegrass has been found with a 400X increase in resistance to Hoelon when compared to susceptible populations.

We have the ability to identify one of these genotypes of resistant ryegrass using a quick bioassay procedure in our lab in Fayetteville, Ark. More research is needed to develop quick assays for identifying the other three that are known.

Currently, the only labeled herbicide option for controlling Hoelon-resistant ryegrass in Arkansas wheat is Finesse applied pre-emergence. This program is good if adequate rainfall occurs after planting and before ryegrass germinates. It will provide excellent suppression under ideal conditions. But, in cases where moisture or application timing is off, it can provide less-than-acceptable control.

A major consideration with this technology is that STS-tolerant soybeans must be planted in a soybean doublecrop program.

Another option for controlling Hoelon-resistant ryegrass, that is in fact labeled, is Beyond herbicide on Clearfield wheat. In our research, Beyond has performed well on resistant ryegrass. However, it has been inconsistent at times, and control varies greatly with environmental conditions, weed size, and application timing. There are currently no acceptable soft red winter wheat varieties available with the Clearfield trait. This technology is being widely used with success in other wheat regions.

Other options are to winter fallow wheat ground for a year and eliminate as many flushes of ryegrass as possible or to plant wheat late in the season after burning down the first one or two flushes of ryegrass.

With that being said, a replacement herbicide for Hoelon is badly needed by many wheat farmers.

Osprey is a new wheat herbicide being developed by Bayer Crop Sciences for use in controlling ryegrass in wheat. It has a different mode of action than Hoelon and has performed as well as or better than Hoelon in most of our university trials. This herbicide represents a true replacement for Hoelon and is not hampered with crop rotational issues.

The EPA has denied Section 18 applications for Osprey the past two seasons due to registration issues. The company anticipates a full Section 3 label in time for use in the fall of 2004.

Another benefit of this herbicide could be the ability to be tank-mixed with a broadleaf weed control partner, such as Harmony Extra or Express. We have not observed the antagonism that is commonly seen when Hoelon is tank-mixed with a broadleaf weed control partner.

Without the generous support of the Arkansas Wheat Promotion Board, our work on Hoelon-resistant ryegrass and other weed control work in wheat would not be possible. We greatly appreciate this support and welcome any feedback from Arkansas wheat producers.

Bob Scott, Nilda Burgos, and Dick Oliver are with the University of Arkansas Cooperative Extension Service.

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